McLain's go-ahead solo homer came in the sixth inning, giving him six long balls for the season. TJ Friedl also homered for the Reds, who snapped a three-game losing streak that followed a 12-game winning streak
The teams, who waited through a rain delay of more than 90 minutes in the eighth inning, have split two games going into Wednesday night's series finale.
Reds starter Andrew Abbott (4-0) limited Baltimore to one run on two hits for six innings. Cincinnati improved to 5-0 in games the rookie has pitched
Lucas Sims, Buck Farmer and Alexis Diaz each worked one inning of relief for the Reds. In the ninth, Diaz hit Ryan O'Hearn with a two-out pitch and then walked Adam Frazier before Austin Hays lined out to end the game. Diaz recorded his 22nd save
Baltimore infielder Jordan Westburg, who was in his second big-league game, went 2-for-3 with a walk. Gunnar Henderson had the only extra-base hit for the Orioles, though he was stranded after a two-out double in the eighth inning
An inning earlier, the Orioles had three batters reach base without a hit against Sims
Abbott struck out eight batters and walked three. Wells racked up seven strikeouts with one walk.
The Orioles began the scoring on Ryan McKenna's sacrifice fly in the second inning. That came after Westburg singled to send Aaron Hicks to third base
McLain doubled in a run in the fourth inning and homered in the sixth. Friedl's fifth homer of the season came as the second batter after the game resumed in the top of the eighth inning.
It was the second night in a row that a game between the teams was interrupted by rain.
--Field Level Media